1. Field of the Invention
The present invention relates to a monocrystalline silicon wafer having good intrinsic getterability, the silicon wafer having a high BMD density in the bulk and a BMD-free layer on its surface. The invention also relates to a method for producing such silicon wafers by heat treatment.
2. Background Art
Silicon single crystals, which are generally produced according to the Czochralski crucible pulling method (CZ method), comprise a range of impurities and defects. The single crystals are sliced into wafers, subjected to a multiplicity of processing steps in order to obtain the desired surface quality and, finally, generally used for the production of electronic components. If special precautions are not taken, then the defects will also be found on the surface of the wafers where they can detrimentally affect the function of the electronic components fabricated thereon.
The following defect types generally occur in silicon wafers as a function of the conditions during production of the silicon single crystal, which are attributable to the presence of point defects i.e. crystal lattice vacancies or interstitial atoms. In the latter sense, interstitial silicon atoms are also referred to as interstitials.
Depending on the preparation or detection method, agglomerates of crystal lattice vacancies are referred to as “flow pattern defects” (FPDs), “gate oxide integrity (GOI) defects” or “crystal originated particles” (COPs). These COPs are octahedral cavities crystallographically perfectly oriented. Their inner wall is typically covered by a 1 to 4 nm thick oxide film which leads to thermal stabilization of the COP, so elimination of this defect by breaking down the oxide film and subsequently injecting interstitials is effectively possible only at temperatures of about 1200° C. and with process times of more than 30 minutes.
The COPs are exposed when sawing the silicon single crystal and subsequently etching and polishing the surface, and are encountered as depressions with a diameter of up to about 200 nm. These depressions lead to problems, for example short circuits on the so-called gate oxide of a CMOS transistor, since defective growth of this surface oxide takes place at this position. Vertical “trenches”, which are produced by anisotropic etching and are part of a typical CMOS memory cell, may also be short circuited by a COP lying between them, or the protective oxide of the memory cell may be compromised. The dependency of the formation of COPs on the ratio v/G of the crystal pull parameters (v=crystal pull rate, G=thermal gradient at the interface between the melt and the growing single crystal) is described for example in V. Voronkov, J. CRYSTAL GROWTH, Vol. 59, p. 625 (1982).
At the interface between the melt and the growing single crystal, crystal lattice vacancies and interstitials are incorporated with an equilibrium concentration. When the crystal cools, recombination of the two point defect types ensures that a concentration higher than the equilibrium concentration (=supersaturation) does not occur either for the interstitials or for the vacancies. This applies so long as there is a significant concentration of interstitials. Thereafter, supersaturation of vacancies builds up. The concentration of vacancies is essentially determined by the parameter v/G (as described in J. CRYSTAL GROWTH, Vol. 59, p. 625 (1982)). In vacancy-rich crystals according to the prior art, aggregation of the vacancies to form COPs takes place after sufficiently high supersaturation is reached. Recent simulations of this COP aggregation show a rise in the COP density with small supersaturations. If the vacancy concentration after recombination is sufficiently small, however, then the aggregation would take place at a temperature significantly lower than 1100° C. With a theoretical aggregation temperature of less than 1000° C., however, the affinity of the vacancies for oxygen is greater than that of vacancies for other vacancies, the effect of which is that vacancies react with interstitial oxygen to form oxygen-vacancy complexes (O2V). A high concentration of O2V is in turn highly conducive to the formation of seeds for oxygen precipitation. The defects thus formed can later be revealed as “oxidation induced stacking faults” (OSF) by subjecting the silicon wafers produced from the single crystal to an oxidation treatment. Those defects already existing before the oxidation treatment, which can be detected as OSFs, will be referred to below as OSF seeds.
Wafers with a high density of OSF seeds present the following disadvantages with respect to reduced component yields: (1) metals are preferentially bound to OSFs (“gettered”), which leads to degradation of the gate oxide due to enhanced formation of nucleation centers for volatile SiO; (2) OSF seeds grow in component-specific thermal processes to form large precipitates, which crucially weaken the silicon matrix due to formation of dislocation loops. This can lead to enhanced deformation (“warp”) of the silicon wafer, which then in turn interferes with the photolithography step used in the CMOS process since the critical minimum structure linewidths are no longer achieved; (3) large oxygen precipitates, which are caused by OSF seeds, cannot generally be eliminated so as to form a BMD-free zone even by a thermal process, and thus they remain as defects in the active component zone. This, as already described for COPs, may lead to short circuits in the trench capacitor memory cell, or may weaken its capacitance.
In silicon single crystals, agglomerates of interstitials lead to dislocation loops with extents of several micrometers, which likewise has a detrimental effect on the function of the components produced there.
The prior art, however, contains ways of substantially avoiding the creation of these defects during crystal growth. It is known for instance that when accurately defined conditions are complied with during the crystal pull, neither agglomerates of crystal lattice vacancies nor agglomerates of interstitials occur. The above-described ratio v/G is of particular importance for this to occur.
Silicon wafers which are substantially free of agglomerates of crystal lattice vacancies and interstitials over the entire surface, generally comprise radial regions in which crystal lattice vacancies are the prevalent point defect type and other radial regions in which interstitials prevail, see for example DE10047345A1 or T. Müller et al., “Precipitation enhancement of so called defect-free Czochralski silicon material”, SOLID STATE PHENOMENA, Vols. 108-109 (December 2005), pp. 11-16. In the latter regions, a subsequent heat treatment generally leads to the formation of only a low concentration of oxygen precipitates (also referred to as BMDs, “bulk micro-defects”). On the one hand this is desirable since oxygen precipitates on the wafer surface can lead to functional impairment or failure of the relevant components. On the other hand, however, oxygen precipitates bind metal impurities which diffuse into the silicon wafer during the production of electronic components. This effect is referred to as the “intrinsic getter effect”, or “IG effect” for short. For this reason, the presence of oxygen precipitates in the interior of the silicon wafer (the “bulk”) is generally desirable. Silicon wafers having regions in which crystal lattice vacancies prevail, and other regions in which interstitials prevail, have a very differently pronounced getterability in these regions owing to the different susceptibility to the formation of oxygen precipitates. For instance, the oxygen precipitates grow stress-free in zones with a vacancy excess by absorption of vacancies, which is not possible in zones with an excess of interstitials. More rapid growth is therefore possible in the vacancy-rich zones, and a mixture of zones with an excess of interstitials and zones with an excess of vacancies is generally undesirable on a wafer.
Another problem, however, occurs during the heat treatment of silicon wafers, in the entire volume of which crystal lattice vacancies are the prevalent point defect type and which at the same time are free of agglomerates of crystal lattice vacancies: If the interstitial oxygen concentration [Oi] is selected to be high enough in order to ensure sufficient BMD formation and therefore sufficient getterability, then interfering OSF seeds are formed during the crystal growth when cooling. Furthermore, this effect also depends on the cooling rate of the crystal rod, since a longer residence time in the temperature zone relevant for the precipitate growth entails commensurately more formation of OSF seeds. If, however, the interstitial oxygen concentration [Oi] is selected to be so low that no OSF seeds are formed during the growth of the crystal, then this leads to no BMD density or a low BMD density which is not enough to achieve a sufficiently large getter effect (for example defined with the aid of the size and density of the BMDs according to Sueoka et al., ELECTROCHEM. SOC. PV 2000-17, p. 164, 2000 or Hölzl et al., Electrochem. Soc. PV 2002-02, p. 602, 2002).